01 - What is Node.js

JavaScript Outside the Browser

Node.js runs JavaScript on your machine instead of in a browser. It uses V8, the same engine Chrome uses, but strips out browser APIs (no document, no window) and adds system-level ones instead (file access, networking, processes).

// This works in Node, not in a browser
const os = require('os');
console.log(os.platform()); // "darwin" (macOS), "linux", or "win32"
console.log(os.cpus().length); // 10 (number of CPU cores)

Running a File

Create a file and run it. That's it.

// hello.js
console.log('Running in Node.js', process.version);
// Running in Node.js v20.11.0

node hello.js

No HTML file. No script tag. No browser. Just node filename.js.

The Event Loop

Node.js is single-threaded but non-blocking. It doesn't wait for slow operations (reading files, network requests). It starts the operation, moves on, and comes back when it's done.

const fs = require('fs');

console.log('1: before read');

fs.readFile('hello.js', 'utf-8', (err, data) => {
  console.log('3: file read done');
});

console.log('2: after read');
// Output:
// 1: before read
// 2: after read
// 3: file read done

The file read doesn't block. Line 2 runs before line 3 because readFile is asynchronous. This is how Node handles thousands of connections with a single thread — the event loop keeps it moving.

Global Objects

Node.js has its own globals instead of browser ones. The process object and Buffer class are the most important.

console.log(__dirname);  // /Users/you/projects/my-app
console.log(__filename); // /Users/you/projects/my-app/hello.js
console.log(process.pid); // 12345 (process ID)

Node REPL

Type node with no arguments to get an interactive shell — the REPL.

$ node
> 2 + 2
4
> 'hello'.toUpperCase()
'HELLO'
> .exit

Useful for quick experiments. Press Ctrl+C twice or type .exit to quit.

Key Takeaways

• Node.js runs JavaScript outside the browser using the V8 engine
• Run files with node filename.js, no browser needed
• Single-threaded but non-blocking: async operations don't freeze execution
• No window or document, instead you get process, __dirname, Buffer
• node with no arguments opens an interactive REPL
• The event loop is why Node can handle many connections on one thread